There is considerable interest in the identification, isolation, and generation of human stem cells. Human stem cells are typically totipotential or pluripotential precursor cells capable of self-renewal and generating a variety of mature human cell lineages. This ability serves as the basis for the cellular differentiation and specialization necessary for organ and tissue development. Recent success at transplanting stem cells have provided new clinical tools to reconstitute and/or supplement bone marrow after myeloablation due to disease, exposure to toxic chemicals, and/or radiation. Further evidence exists that demonstrates that stem cells can be employed to repopulate many, if not all, tissues and restore physiologic and anatomic functionality.
Many different types of mammalian stem cells have been characterized. For example, embryonic stem cells, embryonic germ cells, adult stem cells, and other committed stem cells or progenitor cells are known. In fact, certain stem cells have not only been isolated and characterized, but have also been cultured under conditions to allow a limited degree of differentiation. Because of the tens of millions of possible combinations of HLA types in the population, a basic problem remains, in that it is very difficult to obtain sufficient quantities, populations, and varieties of HLA types of human stem cells which are capable of differentiating into all cell types that can be HLA matched to individual patients. Stem cells of different HLA types are in critically short supply. Due to their importance in the treatment of a wide variety of diseases and conditions, including malignancies, inborn errors of metabolism, hemoglobinopathies, and immunodeficiencies, it would be highly advantageous to have an adequate source of stem cells of various HLA types.
Obtaining sufficient numbers of human stem cells has been problematic for several reasons. First, isolation of normally occurring populations of stem cells in adult tissues has been technically difficult and costly due, in part, to very limited quantities found in blood or tissue. Second, procurement of these cells from embryos or fetal tissue, including aborted fetuses, has raised ethical concerns. Alternative sources that do not require the use of cells procured from embryonic or fetal tissue are therefore essential for further progress in the clinical use of stem cells. There are, however, few viable alternative sources of stem cells, particularly human stem cells, and thus the supply is limited. Furthermore, harvesting of stem cells from alternative sources in adequate amounts for therapeutic and research purposes is generally laborious.
For example, U.S. Pat. No. 5,486,359 discloses human mesenchymal stem cell (HMSC) compositions derived from bone marrow. Homogeneous HMSC compositions are obtained by positive selection of adherent marrow or periosteal cells that are free of markers associated with either hematopoietic cells or differentiated mesenchymal cells. The isolated mesenchymal cell populations display characteristics associated with mesenchymal stem cells, have the ability to regenerate in culture without differentiating, and have the ability to differentiate into specific mesenchymal lineages when either induced in vitro or placed in vivo at the site of damaged tissue. The drawback of such methods, however, is that they first require the invasive and painful harvesting of marrow or periosteal cells from a human donor in order to subsequently isolate HMSCs.
Umbilical cord blood is a known alternative source of mesenchymal stem cells as well as hematopoietic stem cells and progenitor cells. Stem cells from cord blood are routinely cryopreserved for hematopoietic reconstitution, a therapeutic procedure used in bone marrow and other related transplantations (see, e.g., U.S. Pat. Nos. 5,004,681 and 5,192,553). Conventional techniques for the collection of cord blood are based on the use of a needle or cannula, which is used with the aid of gravity to drain cord blood from the placenta (see, e.g., U.S. Pat. Nos. 5,004,681, 5,192,553, 5,372,581, and 5,415,665). The needle or cannula is usually placed in the umbilical vein and the placenta is gently massaged to aid in draining cord blood from the placenta. A major limitation of stem cell procurement from cord blood, however, has been the frequently inadequate volume of cord blood obtained, resulting in insufficient cell numbers to effectively reconstitute bone marrow after transplantation.
Stem cells have the potential to be used in the treatment of a wide variety of diseases and injuries, including nervous system trauma (e.g., spinal cord injury), malignancies, genetic diseases, hemoglobinopathies, and immunodeficiencies. However, stem cells from umbilical cord blood are in critically short supply due to restrictions on their collection, the inadequate numbers of cells typically collected from cord blood, especially if used to treat an adult patient, and the extraordinary cost of establishing a large inventory. As such, there is a strong need in the art for methods of culturing cord blood stem cells in a cell culture system capable of expanding stem cells to a number sufficient for transplantation. There is also a need in the art for methods of enhancing the growth and survival of transplanted stem cells and reducing or delaying stem cell rejection in a recipient. The present invention satisfies these and other needs.